This project will study the maturation of T cell precursors and B cell precursors. These cells, obtained from mouse bone marrow, will be examined for cell-surface antigens and mitogen responsiveness, both as isolated and after treatment with inducers such as thymic hormones and cyclic AMP. The maturation of antigen-reactivity will be followed at the same time. The factors which induce maturation in vivo will be studied, including a possible bone marrow-derived B cell-poietic hormone. The relationship of AKR leukemia cells to T cell precursors will be examined. These studies will serve as a basis for understanding lymphopoiesis in man, especially in abnormal states.